Here's one that's a bit complicated.
When I visit the site, I usually hit the "view unread posts" link. Kudos for being the only homebrew forum that has a "mark all posts read" on the unread-list search page, by the way. TBN and NB don't have that, and HBT's is hidden.
On the unread-list search page, however, if there's more than one page of actual new posts, there's a subtle bug that will make you miss posts. If you click a link on page one and then go to page two of the list, the post that WAS at the top of page two bumps to the bottom of page one (were you to go back to page one, it would be there). That's because the "unread posts" list is regenerated when you hit the "go to page 2" link, and that list has one less element in it (the one you clicked on).
In order to work around this, I just start at the last page and move to the first, instead of starting on page 1.
I'd appreciate it if this didn't happen. There are a number of ways to fix it. You can make it a lot less apparent by making the unread-posts page return more than just 10 or so posts per page; 50 per page would mean I hardly ever had to page around. You can make the unread-posts page return in reverse chronological order, so the oldest new posts are on p1. Or you can change the manner in which the unread-posts page is generated to mean "posts made since my last visit" instead of "posts I haven't explicitly marked read yet", or just find a clever way to maintain the search results list while I page around in them without having to regenerate the list each time. Dunno if caching that search instead of re-running it each time saves on database load or not.